Botez in U.S. Pat. No. 4,347,486, incorporated herein by reference, has disclosed a constricted double heterostructure laser which comprises a body of semiconductor material having a substrate with a pair of spaced, substantially parallel channels in a surface thereof with a mesa therebetween. A first cladding layer overlies the surface of the mesa and partially fills the channels. A guide layer overlies the first cladding layer and an active layer overlies the guide layer. A second cladding layer overlies the active layer. The guide layer has a refractive index less than that of the active layer but larger than that of the first or second cladding layers. The active layer is the recombination region of the laser with the laser light being generated therein in the portion thereof over the mesa. The laser light beam so generated propagates in both the thin active region and, primarily, in the relatively thicker guide layer over the mesa. This laser has excellent selectivity against higher order modes of propagation since the waveguide formed by the active and guide layers in the lateral direction, the direction in the plane of the layers and perpendicular to the axes of the channels, exhibits high loss for those modes. Botez et al. in U.S. Pat. No. 4,383,320, incorporated herein by reference, have disclosed a laser of this type wherein the guide layer has a surface contacting the active layer which has a concave shape in the lateral direction over the mesa and which tapers in increasing thickness in the lateral direction with an active layer which is thickest over the mesa between the channels and which tapers in decreasing thickness in the lateral direction. This structure provides a positive index lateral waveguide for propagation in the active and guide layers, resulting in a reduced threshold current for lasing action and a reduced spontaneous emission near the threshold for laser emission.
The lateral tapering of the active and guide layers provides waveguiding in the lateral direction and is the key to the operation of this laser. This tapering arises from the non-uniform growth rate of layers deposited by liquid-phase epitaxial growth over the channels and mesa. The center-to-center spacing between the pair of channels as well as the individual channel geometry are critical to obtaining the proper curvature of the layers. It would be desirable to provide other means of obtaining this tapering of the active and guide layers.